1. Field of the Invention
The present invention relates to zoom lens systems and particularly, though not exclusively, to a zoom lens system suitable for use as a photographic lens system for an image pickup apparatus.
2. Description of the Related Art
There is a market for a compact and high-resolution zoom lens system with a high zoom ratio photographic optical system for image pickup apparatuses.
To facilitate compactness, a zoom lens system that includes, in order from the object side to the image side, four lens units including a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power, and a fourth lens unit of positive refractive power has been developed and discussed in the following documents: Japanese Patent Laid-Open No. 8-304700, Japanese Patent Laid-Open No. 2000-121941, Japanese Patent Laid-Open No. 2003-295053, Japanese Patent Laid-Open No. 5-027167, Japanese Patent Laid-Open No. 11-202198 (corresponding to U.S. Pat. No. 6,441,968), and Japanese Patent Laid-Open No. 4-43311 (corresponding to U.S. Pat. No. 5,189,558). In the discussed zoom lens system, moving the second lens unit for zooming varies magnification, while moving the fourth lens unit compensates for image plane variations associated with variations in magnification.
A zoom lens system discussed in Japanese Patent Laid-Open No. 5-027167 has a lens configuration that facilitates a ⅓-inch or ¼-inch image pickup device and that can use a plurality of aspheric surfaces to reduce the number of lenses.
In the zoom lens system discussed in Japanese Patent Laid-Open No. 5-027167, a third lens unit includes, in order from the object side to the image side, two positive lens elements and a negative lens element to compensate mainly for spherical aberration. Therefore, in the third lens unit, the positive lens element closest to the object has high refractive power and is convex on both sides. In a positive lens element with such a shape, since there is a substantial difference in optical path length between rays near the optical axis and others near the rim of the positive lens element, it can be difficult to achieve the same level of aberration compensation for different wavelengths. Moreover, since a fourth lens unit is composed of a single positive lens element, compensation for chromatic aberration can be insufficient.
A zoom lens system discussed in Japanese Patent Laid-Open No. 4-43311 (corresponding to U.S. Pat. No. 5,189,558) includes a fourth lens unit that is composed of just one or two positive lens elements. Therefore, compensation for chromatic aberration can be insufficient. Moreover, three positive lens elements included in a third lens unit and one or two positive lens elements included in the fourth lens unit can cause an increase in the overall length of the zoom lens system.
Generally, in a zoom lens system, increasing the refractive power of each lens unit reduces the amount of movement of each lens unit for achieving a predetermined zoom ratio. This means that it is possible to increase the zoom ratio while reducing the overall length of the zoom lens system.
However, simply increasing the refractive power of each lens unit leads to larger variations in aberration during zooming and makes it difficult to provide good optical performance.